Transformers pptxx.ppt

Topics to be covered…
 Introduction / Basics
 Operating Principle
 Classifications of transformer
 Construction
 Transformers parts
 Losses in transformer
 Efficiency
 Parallel operation
 Maintenance

Topics to be covered…
 Testing
 Protection
 3 phase connections / vector groups
 Transformers in IFFCO Kalol plant
 Frequently asked questions

Introduction
 A transformer is a device that converts ac electric
energy at one voltage level to ac electric energy at
another voltage level at same frequency.
 It consists of one or more coil(s) of wire wrapped
around a common ferromagnetic core.
 These coils are usually not connected electrically
together.
 However, they are connected through the common
magnetic flux confined to the core. (Mutual Flux)

Introduction
 Assuming that the transformer has at least two
windings, one of them (primary) is connected to a
source of AC power; the other (secondary) is
connected to the loads.

Operating Principle
 It is based on principle of mutual induction,
according to which an e.m.f. is induced in a coil
when current in the neighbouring coil changes.

Classification of Transformers
 based upon Number of Phases
• Single Phase Transformers
• Poly-Phase Transformers
 based on construction
 based on Function
• Step up transformer
• Step down transformer (Distribution transformer )

 Instrument transformers
• Current transformers
• Potential transformers
 Welding transformers

 based on Type of cooling
 based on location
• Indoor transformer
• Outdoor transformer

Construction
 Shell type
Windings are wrapped around the centre leg of a
laminated core.

Construction
 Core type
Windings are wrapped around two sides of a
laminated square core.

Some Constructional Facts
 The basic transformer is formed from two coils that
are usually wound on a common core to provide a
path for the magnetic field lines.
 The cores of transformers are laminated in order to
reduce eddy current losses.
 These laminations are made of transformer grade
steel containing 3-5% silicon, which increases the
resistivity of the core, thereby reducing the eddy
current core loss.

 The steel used for transformer core is generally
cold rolled grain oriented steel as it has the
maximum permeability and minimum specific iron
loss.
 The joints at the junctions of different strips of
lamination introduce air gaps which result in
increased magnetizing current. To reduce the
effective air gap, joints in adjacent laminations are
overlapped.
Some Constructional Facts

Direction of Windings
 The direction of the windings determines the
polarity of the voltage across the secondary
winding with respect to the voltage across the
primary.
 Phase dots are sometimes used to indicate
polarities.

 Conservator
• It is an Expansion vessel.
• It maintains oil in the Transformer above a
Minimum level.
• It has a Magnetic Oil Level Gage.
• It can give an alarm if the oil level falls below
the limit.
Transformers parts

• This usually has oil level indicators
• Main Conservator Tank can have a Bellow
• It has an oil filling provision
• It has an oil drain valve
• Provision is there for connecting a Breather
Transformers parts

 Buchholz Relay
• The purpose of such devices is to disconnect
faulty apparatus before large scale damage
caused by a fault to the apparatus or to other
connected apparatus.
• Such devices generally respond to a change in
the current or pressure arising from the faults
and are used for either signalling or tripping
the circuits.
Transformers parts

 Buchholz Relay
• In the event of fault in an oil filled transformer
gas is generated, due to which Buchholz relay
gives warning of developing fault.
• Buchholz relay is provided with two elements
one for minor faults (gives alarm) and other for
major faults (tripping). The alarm elements
operate after a specific volume gets
accumulated in the relay.
Transformers parts

Transformers parts
 Breather
• Prevents Moisture Ingress.
• Connected to Conservator Tank
• Silica Gel is Blue when Dry; Pink when moist
• Oil Seal provides a Trap for moisture before
passing through silica Gel

 WTI
 OTI
 Radiator
 MOG
 Marshaling box
 Tap changer
Transformers parts

Losses in transformer
 Core loss or Iron loss (Pi)
It is the sum of hysteresis (Ph) and eddy current
(Pe) loss. It is constant for a machine.
1. Eddy Current Loss
where, Ke = proportionality constant
f = frequency
Bm = maximum flux density in the core
2. Hysteresis Loss
where, Kh = proportionality constant
x = Steinmetz constant : varies from 1.6 to 2.5

 Copper Losses
Also known as losses
Cu loss = Primary winding Cu loss + Secondary
winding Cu loss
Losses in transformer

Efficiency of transformer
Efficiency 100
*
2
2
2
2
2
2


Cos
I
V
Losses
Cos
I
V



Maximum Transformer Efficiency
 The efficiency varies as with respect to two
independent quantities namely, current and power
factor.
 Thus at any particular power factor, the efficiency
is maximum if core loss = copper loss .This can be
obtained by differentiating the expression of
efficiency with respect to I2 assuming power factor,
and all the voltages constant.

 At any particular I2 maximum efficiency happens
at unity power factor.
 This can be obtained by differentiating the
expression of efficiency with respect to power
factor, and assuming I2 and all the voltages
constant.
 Maximum efficiency happens when both these
conditions are satisfied.

100
0
% full load current
pf=1
pf= 0.8
pf= 0.6
At this load current
core loss = copper loss
Maximum efficiency point


Parallel operation of transformers
 The polarities of the transformers must be same.
 Identical primary and secondary voltage ratings.
 Impedances inversely proportional to the kVA
ratings.
 Identical X/R ratios in the transformer
impedances.
 The phase sequence must be same.
 The phase shift between primary and secondary
voltages must be the same for all transformers
which are to be connected in parallel.

Maintenance of transformers
 Inspection of primary and secondary cable boxes,
end termination, checking and tightening of
connections.
 Recommended Tests & Measurement of
parameters like Insulation resistance, BDV of
transformer oil are carried out on each transformer.
 Alarm & tripping contacts of Buchholz relay and
MOG are checked.

 Condition of silica gel is checked. Accordingly
discharged silica gel is recharged.
 Oil leakages from the transformers are attended
and damaged gaskets are replaced
Maintenance of transformers

Testing of transformer
 Routine tests
The routine tests are conducted to ensure the
particular transformer is free from manufacturing
defects. ( To be carried out on each job)
• Measurement of winding resistance
• Measurement of insulation resistance
• Separate source voltage withstand test (high
voltage tests on HV & LV)

• HV high voltage test : LV winding connected
together and earthed. HV winding connected
together and given 28 KV ( for 11KV transformer)
for 1 minute.
• LV high voltage test : hv winding connected
together and earthed. LV winding connected
together and given 3 KV for 1 minute.
• Induced over voltage withstand test (DVDF test)
for a 11KV/433V transformer,866 volts are applied
at the 433V winding with the help of a generator
for 1 minute. This induces 22KV on 11KV side.
The frequency of the 866V supply is also increased
to 100HZ.

• Measurement of voltage ratio
• Measurement of no load loss & current.
• Measurement of load loss &
impedance.(Efficiency & regulation)
• Vector group verification
• Oil BDV test.
• Tests on OLTC (if attached)

 Vector group Dyn11 verification
RY=Rn+Yn (400=7+393)
Yy=Yb (388=389)
By>Bb (399>389)

 Type test
The type tests are conducted to check the
particular design parameters.
• Lightening Impulse test.
• Temperature rise test

 Special tests
The special tests are conducted depending upon the
need for particular test-as per site conditions.
The special tests have to be agreed between the
purchaser and supplier. Hence purchaser should
clearly specify conducting special tests in the P.O.
• Dynamic short circuit test
• Measurement of Zero sequence impedance

• Measurement of harmonics
• Partial discharge measurement
• Measurement of tan delta
• Measurement of noise level

Protection of transformer
 The best way of protecting a transformer is to have
good preventive maintenance schedule.
 Oil Temperature Indicators.
 Winding Temperature indicators.
 Buchholz Relay.
 Magnetic Oil level Gauge.
 Explosion Vent.

 HT fuse & D.O. fuse.
 LT circuit breaker.
 HT Circuit breaker with Over load, Earth Fault
relay tripping.
 PRV for OLTC
 Breather
 Fire push button
Protection of transformer

3 phase connections / vector groups
 There are many combinations in which HV and LV
windings of transformers employed in 3-phase
systems may be connected.
 3-phase transformers are divided into four main
groups according to the phase difference between
the corresponding line voltages on the HV and LV
sides.

 Delta-delta connection

 Star - star connection

 Delta-star connection

 Star-delta connection

Frequently asked questions
 Why transformer ratings in kva
 Why transformer does not rotate
 Operation of transformer when applied DC
 Why tap changer on HV side

Any query?
•If any mistake or wrong information given here,
please rectify/correct me.

Transformers pptxx.ppt

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